Video monitoring, which is an important part of a security protection system, has been widely used in many occasions because of its visuality, accuracy, promptness and richness in information contents. In recent years, with the rapid development of computers, networks and image processing and transmission technologies, the trend of popularization of video monitoring is becoming more and more apparent.
Passive Infrared (PIR) technology can be applied to a video surveillance device, and a video surveillance device using the PIR technology is called a PIR camera. In an environment of low illumination, a PIR camera may switch to a black/white mode and automatically turn on an infrared lamp. When a motion detection mechanism or a PIR detector of the PIR camera detects that a monitored target is in a distance from the camera within a particular range, the PIR camera may automatically switch to a color mode to take a photograph or record a video.
After switching to the color mode, the PIR camera needs to re-perform an exposure calculation. When the exposure calculation is re-performed, an exposure parameter may be adjusted from the right end or some special point of an exposure line as a starting point. The exposure line refers to a sequence in which exposure elements such as shutter speed, aperture, gain and the like are adjusted during an exposure adjustment. As shown in FIG. 1, a point Pa corresponds to an exposure parameter that should be used in a color mode, and the exposure parameter may be adjusted by starting from the right end or a special point Pb of the exposure line to gradually approximate to the point Pa through light metering and exposure calculation. However, because the light metering and the exposure calculation may take time, a monitored target may leave a monitoring range before the exposure calculation is completed, resulting in that the monitored target cannot be captured a color image in time.
Therefore, it is pretty important to estimate an exposure parameter to be used before a PIR camera switches to a color mode.